Generation and Transport of Bubbles in Intense Ultrasonic Fields

This paper examines cavitation phenomena generated by a 500 kHz, high-intensity focused ultrasonic (HIFU) beam, with pressure amplitude in the focal zone of up to 19 atm, in quiescent water and at the exit of a jet. The pressure field and cavitation are visualized using high-speed digital in-line holography. The spatial distribution of the acoustic pressure is determined form the variations in the water density and refractive index. A partial standing wave is generated by the reflection of the sound from the wall of the test chamber. Several cavitation phenomena are observed. At low to moderate sound levels, bubbly layers form in the pressure nodes of the standing wave, in the periphery of the focal zone. At high sound levels, clouds of vapor bubbles are generated in the antinodes, and migrate in the direction of the acoustic beam at speeds in the 1–4 m/s range. Both the cloud size and velocity oscillate, with the size peaking in the nodes and the velocity in the antinodes. A model for the cloud dynamics shows that the periodic velocity variation is dominated by the balance between the primary Bjerknes force and the drag. The secondary Bjerknes force involving interactions among the bubbles within the cloud is a likely cause for the size oscillations.